For example, in a thermoplastic synthetic resin raw material (hereinafter, referred to as simply a synthetic resin raw material) such as plastic or rubber containing a liquid component such as water, the synthetic resin raw material is molded into a synthetic resin molding product after the liquid component is removed. When the liquid component is removed from the synthetic resin raw material, in the related art, a co-rotating intermeshing type twin-screw extruder (hereinafter, referred to as simply an extruder) is used.
The extruder used when the liquid component is removed from the synthetic resin raw material is described with reference to FIGS. 17(a) and 17(b). FIGS. 17(a) and 17(b) are an upper cross-sectional view and a side cross-sectional view showing an outline of a general extruder.
As shown in FIGS. 17(a) and 17(b), an extruder 1 includes a cylinder 2 having a cylindrical shape, and a die 3 provided on one end of the cylinder 2. The cylinder 2 includes a raw material supply port 4 for inputting a synthetic resin raw material into the cylinder 2, and a liquid discharge port 5 for discharging a liquid component removed from the synthetic resin raw material from the inner portion of the cylinder 2.
In addition, the extruder 1 includes a pair of screws 6 which is rotatably accommodated in the cylinder 2. The pair of screws 6 is disposed such that rotary axes R of the screws 6 are parallel with each other.
Each screw 6 includes a transport portion 6a which transports the synthetic resin raw material in the cylinder 2 from a raw material supply port 4 in an X1 direction toward the die 3, and a throttling portion 6b which kneads the synthetic resin raw material and removes the liquid component from the synthetic resin raw material. The transport portion 6a and the throttling portion 6b are alternately disposed in the X1 direction, the synthetic resin raw material supplied from the raw material supply port 4 is transported to the throttling portion 6b by the transport portion 6a, passes through the throttling portion 6b, and is transported to the die 3.
When the liquid component of the synthetic resin raw material passes through the throttling portion 6b, the liquid component is removed from the synthetic resin raw material. The synthetic resin raw material, in which the liquid component is removed, is transported from the throttling portion 6b toward the die 3, and is extruded from the inner portion of the cylinder 2 while being molded to a predetermined shape by the die 3.
The liquid component removed from the synthetic resin raw material flows through the transport portion 6a from the throttling portion 6b in an X2 direction opposite to the X1 direction. The liquid discharge port 5 is formed on the X2 direction side of the throttling portion 6b, and the liquid component removed from the synthetic resin raw material is discharged from the inner portion of the cylinder 2 through the liquid discharge port 5.
As one of factors which determine a capability (hereinafter, also referred to as a transportation capability) for transporting the synthetic resin raw material of the screw 6 in the X1 direction or a capability (hereinafter, also referred to as a draining capability) for causing the liquid component removed from the synthetic resin raw material to flow in the X2 direction, there is a structure of the transport portion 6a. Structures of various transport portions 6a capable of improving the transportation capability of the screw 6 or the draining capability have been suggested (for example, PTL 1).
The structure of a transport portion referred to as an angle flight screw piece which is disclosed in PTL 1 will be described using FIGS. 18(a) and 18(b). FIG. 18(a) is an outline view of a pair of angle flight screw pieces, and FIG. 18(b) is a cross-sectional view taken along line A-A of FIG. 18(a).
As shown in FIGS. 18(a) and 18(b), the angle flight screw piece 7 includes an outer circumferential surface 9 in which a groove 8 is spirally formed. In the inner surface of the groove 8, a first surface 8a facing in the X1 direction and a second surface 8b facing in the X2 direction are erected to be approximately perpendicular with respect to an axis center of the angle flight screw piece 7 from a bottom surface 8c of the groove 8.
The synthetic resin raw material input into the cylinder 2 (refer to FIGS. 17(a) and 17(b)) enters the inner portion of the groove 8. According to a rotation of the angle flight screw piece 7 the first surface 8a presses the synthetic resin raw material in the X1 direction. As a result, the synthetic resin raw material is transported in the X1 direction. The liquid component removed from the synthetic resin raw material flows in the X2 direction via a space in the groove 8 where the synthetic resin raw material is not present.
Since the first surface and the second surfaces 8a and 8b are parallel with each other in the angle flight screw piece 7, a volume of the groove 8 is relatively large. Accordingly, a sufficient space through which the synthetic resin raw material or the liquid component moves is easily secured. As a result, the liquid component easily flows in the X2 direction and the draining capability of the screw 6 (refer to FIGS. 17(a) and 17(b)) is improved.
However, in the angle flight screw piece 7, when the synthetic resin raw material is transported, a portion of the synthetic resin raw material may adhere to the first surface 8a. The synthetic resin raw material adhering to the first surface 8a is not easily transported in the X1 direction, and thus, the transportation capability of the screw 6 (refer to FIGS. 17(a) and 17(b)) is decreased. In PTL 1, a structure of a transport portion referred to as a ball flight screw piece, which has a higher transportation capability than the angle flight screw piece, is also disclosed.
FIG. 19(a) is an outline view of a pair of ball flight screw pieces, and FIG. 19(b) is a cross-sectional view taken along line B-B of FIG. 19(a). Moreover, the same reference numerals are assigned to the same components as those shown in FIGS. 18(a) and 18(b), and the components are simply described.
The first and second surfaces 8a and 8b of one ball flight screw piece 10 and the outer circumferential surface 9 of the other ball flight screw piece 10 are curved to come into contact with each other. Accordingly, the pair of ball flight screw pieces 10 are rotated in the same direction, and the first and second surfaces 8a and 8b of the one ball flight screw piece 10 are in slide-contact with the outer circumferential surface 9 of the other ball flight screw piece 10.
In the pair of ball flight screw pieces 10, the outer circumferential surface 9 of the other ball flight screw piece 10 is in slide-contact with the first surface 8a of the one ball flight screw piece 10. Accordingly, the synthetic resin raw material adhering to the first surface 8a of the one ball flight screw piece 10 is scraped off.
The other ball flight screw piece 10 also has a configuration similar to the one ball flight screw piece 10. That is, the pair of ball flight screw pieces 10 can transport the synthetic resin raw material without the adhesion of the synthetic resin raw material on the first surface 8a of each ball flight screw piece 10. Accordingly, in the ball flight screw pieces 10, a relatively high transportation capability can be obtained.